1. Field of the Invention
The present invention relates to a technique for performing image formation processing by an electrophotographic method, such as a laser printer and a copy machine.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a copy machine and a laser beam printer, forms an image by performing the following process. First, a surface of a photosensitive member is charged by a charging apparatus. Then, an electrostatic latent image is formed on the photosensitive member by exposing the charged photosensitive member to a light beam to change an electric potential on the surface of the photosensitive member from a charge potential. The formed electrostatic latent image is developed as a toner image by a development apparatus, and the developed toner image is transferred to a recording medium such as paper. The toner image transferred on the recording medium is fixed thereon by a fixing apparatus.
It is impossible to form a photosensitive layer with a uniform thickness (hereinafter referred to as “film thickness”) on a photosensitive member due to the limitation of manufacturing accuracy. Since the film thickness of the photosensitive layer affects a potential change characteristic at the time of charging and exposing the photosensitive member, a non-uniform film thickness makes it impossible for the photosensitive member to have a uniform surface potential when the photosensitive member is charged and when the photosensitive member is exposed to a light beam of a constant light amount. A non-uniform film thickness of the photosensitive member makes it impossible to achieve uniformity in the surface potential of the electrostatic latent image at positions having different film thicknesses even if the photosensitive member is charged and exposed under the same condition. A non-uniform surface potential leads to a non-uniform toner adhesion amount when the electrostatic latent image is developed, so that image density may vary at different positions even if the charging and exposure are performed under the same condition, resulting in density unevenness in an output image.
Japanese Patent Application Laid-Open No. 2004-223716 discusses a technique (shading correction) for correcting density unevenness by correcting a variation in a surface potential of a photosensitive member due to unevenness of a film thickness of the photosensitive member. An image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-223716 includes a memory for storing light amount correction data according to the film thickness. The image forming apparatus locates a position to be exposed to a light beam on the photosensitive member during image formation, reads out the light amount correction data from the memory according to the exposure position, and controls the light amount of the light beam based on the light amount correction data.
The photosensitive member is provided with a reference mark indicating a rotation reference position thereof to locate an exposure position, and the image forming apparatus can start counting of a reference clock when the reference mark passes through a rotation reference position sensor. The image forming apparatus locates, based on the count value, an exposure position to a light beam in the rotational direction of the photosensitive member. Further, the image forming apparatus includes a synchronization sensor for adjusting an image writing start position in the rotational axial direction of the photosensitive member in a region scanned by a light beam. The image forming apparatus starts counting by another counter when the synchronization sensor outputs a synchronization signal, and locates an exposure position in the rotational axial direction based on the count value. The light amount correction data is read out from the memory according to the exposure position located based on both of the count values.
Once image data is input, the photosensitive member is controlled to be accelerated so that the rotational speed thereof reaches a predetermined rotational speed, and once the rotational speed thereof reaches the predetermined rotational speed, the photosensitive member is controlled to rotate at a constant speed. The above described image forming apparatus is set in a state ready for image formation (exposure), when the speed control is switched from the acceleration control to the constant speed control, and the rotational reference position sensor detects the reference mark (home position mark, hereinafter referred to as “HP mark”).
However, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-223716 includes the following problem. In a precise sense, the photosensitive member does not rotate at a constant speed. If the rotational speed is changed, the light amount correction data corresponding to the exposure position to a light beam may be unable to be read out. More specifically, for example, it is assumed that the light amount correction data corresponding to the count value “10” is read out from the memory while the photosensitive member rotates at a higher speed than a predetermined rotational speed. In this case, however, an actual exposure position would be a position corresponding to the count value “11”, since the rotational speed of the photosensitive member is higher than the predetermined rotational speed. When the corresponding relationship cannot be established between the read light amount correction data and the exposure position in this way, it is impossible to execute accurate shading correction.
If an encoder is disposed at the photosensitive member for counting an output from the encoder, the light amount correction data can be read out based on the count value, and the corresponding relationship can be maintained between an exposure position and the read light amount correction data if the rotational speed of the photosensitive member is changed. However, a new encoder in addition to the rotational reference position sensor for detecting the HP mark is required, so that the manufacturing cost of the image forming apparatus will increase.